1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a beacon transmission in a wireless network, and more particularly, to a beacon transmission which overcomes a service hole problem which occurs in a hard time beacon scheme.
2. Description of Related Art
Wireless sensor networks have received a great deal of attention for several years, since wireless sensor networks envision many useful applications ranging from environmental monitoring to home automation. One essential networking function in distributed sensor networks is broadcasting, since distributed sensor networks support device discoveries, route discoveries, and network maintenance. In order to keep network information updated, a broadcast message is transmitted periodically. Based on time characteristics, broadcasting is classified into three categories: hello message, soft-time beacon, and hard-time beacon. The hard-time beacon is more beneficial to low-power and low-rate networks, as the hard-time beacon may define strict time periods for wake-up and sleep.
FIG. 1 is a diagram illustrating a hard-time beacon scheme according to an Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standard.
Communication apparatuses in the IEEE 802.15.4 network transmit a beacon at a scheduled time and start a superframe. A communication apparatus A and a communication apparatus B transmit the beacon at regular intervals, as illustrated in FIG. 1. In a hard-time beacon scheme, the beacon is periodically broadcasted. In FIG. 1, a beacon interval 130 indicates a time interval from a beacon slot to a subsequent beacon slot. The communication apparatuses in the hard-time beacon scheme transmit the beacon at fixed beacon intervals. When the beacon is transmitted at the scheduled time, beacons transmitted by different communication apparatuses may collide. As an example, in FIG. 1, when the communication apparatus A and the communication apparatus B transmit during a beacon slot 111 and a beacon slot 124 at a same time, respectively, a beacon collision may occur.
Unlike data packet collision, a periodic hard-time beacon collision is permanent in nature, since a beacon period is the same for all nodes in the network. If the beacons from two or more adjacent nodes collide, any nodes in an overlapped transmission area are not able to receive the beacon. This area is called a service hole, and such problem is called a service hole problem.
FIG. 2 is a diagram illustrating a service hole problem according to a conventional art.
To overcome the service hole problem, a well designed beacon scheduling is required. For this, nodes 201, 202, 203, 204, and 205 in a network may exchange information. In FIG. 2, when the nodes 201, 202, 203, 204, and 205 exchange hop information, the node 202 and the node 204 may not obtain hop information of the node 204 and the node 202, although the node 202 and the node 204 have an overlapped area 206. In this case, when the node 202 and the node 204 transmit a beacon by using a hard-time beacon scheme, and once a beacon collision occurs, the beacon collision becomes permanent and the overlapped area 206 becomes a service hole. Even when In hop information having at least 2 hops is exchanged, the service hole may still exist. In the service hole, a new device may not join the network.
Thus, a beacon transmission method and apparatus thereof which overcome a service hole problem which occurs in a hard time beacon scheme is needed.